1. Field of the Invention
The present invention relates generally to a method and apparatus for machining or otherwise processing a workpiece such as an electronic substrate with the aid of a high energy beam and a tracking beam which corrects astigmatism, bow distortion and field curvature, and more particularly to a system for evaluating and aligning a laser processing mechanism which incorporates an astigmatically corrected catadioptric laser scanner.
2. Description of Related Art
In the current manufacture of multilayer ceramic (MLC) substrates for integrated circuit semiconductor package structures, a plurality of green ceramic sheets is formed by doctor blading a slurry containing a resin binder, a particulate ceramic material, solvents, and a plasticizer, drying the doctor bladed sheet and cutting it into appropriate sized sheets. Via holes are then mechanically punched for forming electrical interconnections through the sheet. Electrically conductive paste is deposited in the holes, and in appropriate patterns on the surface of the sheets, the sheets stacked and laminated, and the assembly subsequently fired at an appropriate sintering temperature. Punching the via holes in ceramic sheets presents formidable engineering problems in view of the small size, high density, and the complex patterns of the via holes. Apparatus used to perform these operations are described in IBM Technical Disclosure Bulletin (TDB) Vol. 13, No. 4, Feb. 13, 1971 P. 2536; IBM TDB Vol. 16, No. 12, May 1974 P. 3933; IBM TDB Vol. 20. No. 4, Sep. 1977, P.1379; and U.S. Pat. Nos. 4,425,829; 3,730,039; and 4,821,614, the disclosures of which are incorporated by reference herein.
The mechanical punching technology currently used to manufacture MLC substrates has several limitations. The aspect ratio of a hole should theoretically be no less than one, that is the diameter should not be less than the thickness of the sheet to be punched. As the miniaturization of electronic devices continues, the requirement that smaller via holes be used increases. A certain minimum sheet thickness is necessary, however, for the mechanical integrity of the structure.
In addition to requiring smaller diameter holes, future electronics devices will require that the holes be spaced closer together. Use of a mechanical punch at these geometries causes greatly increased embossing of the green sheet, which can greatly distort the via pattern.
High energy beams, including lasers, have been used to machine a variety of workpieces and this activity has been widely reported in the literature. Examples of using lasers to drill holes in electronic substrates are described in U.S. Pat. No. 4,544,442 and in U.S. Pat. No. 4,789,770. An apparatus combining laser machining with a mechanical punch is shown in U.S. Pat. No. 4,201,905.
A rotary metal removing operation such as reaming is not adaptable to extremely small hole diameters of high production rates, however. These types (small hole diameter or high production rates) of applications require a pulsed, high power laser which, if not accurately positioned, can improperly machine or ruin the workpiece. Because of this pulsing, it is not possible to use the beam to determine the machining position, since any possible damage will have already occurred by the time the position is determined. A low energy continuous laser, such as a HeNe laser, collinear with the high energy laser has been used to determine the position of the high energy laser. A control system which allows implementation of this idea is described in U.S. Pat. No. 3,902,036. Use of a similar control system for ophthalmic applications in described in U.S. Pat. No. 4,520,816.
A particularly useful apparatus and method for accurately and rapidly positioning and machining a workpiece comprising an electronic substrate is disclosed in U.S. Pat. No. 5,168,454, assigned to the assignee of this application. A low power HeNe laser is joined collinearly to the optical path of a high power pulsed Nd:YAG laser, and the collinear beam then scans one axis of the workpiece. The low power beam is partially split off to a location-determining device before final deflection of the beams to the workpiece. Deflection in the second axis is achieved by linearly moving the workpiece so that the beam will impinge upon the desired location of the workpiece.
Given the requirement for smaller and more closely spaced features in MLC substrates, a need exists for an apparatus and a method which use advanced technology to manufacture substrates with the required geometries. This apparatus and method must be able to accurately and rapidly machine features in these substrates in order to provide the necessary feature geometries and yet remain competitive with existing mechanical devices such as the multiple-punch apparatus described in U.S. Pat. No. 4,425,829.
A need also exists for an apparatus and method for verifying that the features machined in the workpiece are correctly located. Although a separate apparatus for performing this function has been previously described, for example in U.S. Pat. No. 4,555,798, a need exists for an apparatus and method which are capable of being integrated into the machining apparatus and method. Alignment of the optics in scanning systems are also critical, and many prior art systems are not practical for use in integrated machining systems.
Scanning systems using lasers are used in many applications. Laser scanners are part of a growing multi-billion dollar industry. For example, laser light could be scanned to drill holes in a semiconductor substrate to create micro-circuitry, or it could be used to scribe alpha-numerics on a part or be used to read bar codes or used in laser printers.
The laser scanning systems can be basically classified into three types: Objective Scanners, Pre-Objective Scanners, and Post-Objective Scanners.
Objective Scanners 19, as shown in FIG. 1A and 1B, are the types of scanners which use a simple lens 10, to focus a beam of light 25, such as a laser beam 25, onto a workpiece or a part 12. The focused laser beam 25, is then scanned over the part 12, by moving the part 12, as shown in FIG. 1B. A major advantage of Objective Scanners is that the optics are less complex. Major disadvantages of Objective Scanners are their slower scan speeds and requirements of complex strategies to move the lens or the part.
Pre-Objective Scanners 29, as shown in FIG. 2, are the types of scanners that have a moving, mirrored surface 22, typically a galvanometer or a rotating mirrored polygon, which reflects the laser beam 25, into a lens 20. The lens 20, then focuses the laser beam 25, onto a part 12, at location 23. When the mirrored surface changes its angle, mirrored surface 22', directs the laser beam 25, at a different angle and position into the lens 20. The lens 20, then focuses the beam 25, to another point 27, on the part 12, as can be clearly seen in FIG. 2. Generally, the lenses 20, in a Pre-Objective scanning system 29, are complex and expensive. major advantages of Pre-Objective Scanners 29, are their high scan speeds and their ability to have a flat-field image. However, the major disadvantages of Pre-Objective Scanners 29, are that the lenses are very complex, the lenses are not telecentric (a telecentric lens allows the center for the scanned beam of light to impinge the work surface orthogonally throughout the scan) unless the lens is very large, the system is complex, color correction is very difficult and all these features make the system very expensive.
Post-Objective Scanners 39, as shown in FIGS. 3A (side view) and 3B (top view), are the types of scanners that have a moving mirrored surface 22, usually a galvanometer or a rotating mirrored polygon, after a focusing objective lens 30. The light or laser beam 25, after passing through a lens will also be referred to a light or laser beam 125. The laser beam 25, first passes through the lens 30, which starts to bring the laser beam 125, to a focus. The laser beam 125, is interrupted and reflected by the galvanometer or the mirrored surface 22, to focus on the surface of the part 12, at point 35. When the scanning mechanism changes its angle it redirects the focus of the beam 125, as illustrated in FIG. 3A, to either spot 33 or 37, on an imaginary curve or arc 31. The beam 125, is perfectly focused on the arc 31, but it is out of focus at points or spots 34 and 38, on the flat surface 32, of the workpiece or part 12. For the Post-Objective Scanner 39, in FIG. 3A, the laser beam 125, is folded by the galvanometer 22, 90 degrees in the X-Y plane, so that the laser beam focuses at point 35. When the galvanometer 22, scans the beam 125, it rotates about an axis 135, which lies in the center of the focusing beam 125, on the surface of the galvanometer 22, and in the direction of the Z-axis. The galvanometer 22, moves so that it intersects the laser beam 125, at an angle which is not 90 degrees in the X-Y plane. This causes the focused beam to scan along the arc 31. FIG. 3B, shows this Post-Objective Scanner 39, from a view that looks down on the scanned arc 31. Although the scanned arc 31, is out of focus at the edge of the scan it travels a straight path in the X-Z plane. Generally, the lenses 30, in a Post-Objective scanning system 39, are simple and inexpensive. major advantages of Post-Objective scanners are their scan speeds, simplicity of the object lens, color correctability and their ability to be designed for more wavelengths. Major disadvantages include the fact that the image field is out of focus at the edges of the scanned field or not in focus throughout the scanned distance.
Prior art systems for alignment of laser scanners, particularly telecentric scanners, have been problematic. It continues to be desirable to evaluate the following performance parameters in telecentric scanning systems:
axial image quality, e.g., the degree of spherical aberration, axial astigmatism or axial coma PA1 field image quality, e.g., the degree of astigmatism or coma PA1 field curvature, i.e., the degree of defocusing of the spot as the scan length increases PA1 direction of field curvature, i.e., the degree to which the best focus approaches or recedes from the lens as the scan length is increased PA1 slant of the focused scan, i.e., identification of the plane in which the best focus of the scan resides PA1 straightness of the scan PA1 telecentricity
Prior art methods permit qualification of all of these parameters with different levels of precision and different measurement methods for each parameter. For example, the prior art method of measuring axial image quality involves making an interferogram on the optical axis of the scan system. Another method creates an image of a point source of light with the scan lens to evaluate the blur in that image to determine the aberrations present. Field image quality can be measured on a nodal slide measurement system, for example the system sold by Eidolon Corporation of Natick, Mass. This device allows the user to image a point source with the scan lens to tilt the scan lens to simulate a field angle and permit evaluation of off-axis or "field" points. Field curvature can be evaluated by measuring the size of the focused spot over the length of the scan upon the work surface and by measuring the distance the image plane must be moved to regain the best focus position off axis. Slant of the focused scan can be measured in a similar manner. Straightness of the scanned beam can be measured in a somewhat crude manner relative to a straight edge placed upon the work surface. Telecentricity is typically measured by measuring the changes to the length of the scanned line upon the work surface as the system focus is changed. If the system is truly telecentric, the length of the line will increase due only to defocusing. This is a relatively painstaking process and may not be very precise.
These prior art systems and methods have been able to measure these characteristics in one way or another, but a different set-up has been required for each of the measurements, and the-measurements have not always been precise.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus and a method that will provide an alignment method to create an astigmatically corrected catadioptric laser scanner.
A further object of the invention is to provide a means for minimizing bow distortion, astigmatism, and field curvature resulting from a Post-Objective Scanner system.
It is yet another object of the present invention to provide a scanner system that uses very inexpensive lenses and mirrors.
It is a further object of this invention to provide a beam that is telecentric, i.e., the center of the focusing beam is perpendicular to the part at all points along the scan.
It is still yet another object of the invention is to provide an optical system which is inexpensively adaptable to different wavelengths of light.
It is a further object of this invention to provide an optical scanning system in which the length of the scanned line is directly proportional to the angular change of the moving mirrored surface. This is the result that is sought for a so-called F-theta lens.
It is another object of the present invention to provide a method and apparatus for producing and machining microelectronic components and substrates with densely packed small feature connections.
It is a further object of the present invention to provide a method and apparatus for producing and machining microelectronic components and substrates which permits high throughput (productivity) and low defect rates.
It is yet another object of the present invention to provide a method and apparatus for producing and machining microelectronic components and substrates which may be utilized in "just-in-time" systems and is adaptable to meet future production and growth requirements.
It is still another object of the present invention to provide an improved method and apparatus for aligning, adjusting, tracking, controlling, and verifying machining in a system utilizing (laser) machining beams.
It is a further object of the present invention to provide a method of aligning and evaluating performance parameters in a scanning system which permits precise measurements of multiple parameters with a single apparatus set-up, and which permits adjustment of the scanning system during evaluation to optimize performance.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.